The ballistic performance of ceramic armor improves as the porosity within the ceramic body decreases to zero. Further, as porosity is often not uniformly distributed, its presence leads to inconsistent and unpredictable ballistic response to high velocity projectiles. Thus, for an armor application, ideally, the entire body of a ceramic body is uniformly pore free, rather than having an overall high relative density which would not preclude sporadic localized porosity.
For a pore-free ceramic body, decreased grain size correlates to increased hardness (and correspondingly improved ballistic stopping power) based on a Hall-Fetch relationship. However, simultaneously achieving small grain size and a low volume percent porosity has proven difficult in many ceramic systems because higher sintering temperatures and/or longer sintering times to reduce porosity will usually foster exaggerated grain growth.
Silicon carbide (SiC) is a well known ceramic used as the strike face in armor systems. A typical armor may be made from a SiC body, which has been fabricated through a solid state sintering of a SiC green body made from SiC particles/powder. Carbon and boron carbide (B4C) are typical additives used to facilitate solid-state sintering of SiC. It is well known that a silica (SiO2) passivation coating is invariably formed on SiC particles during synthesis, communition, and handling. To remove silica, SiC particles are evenly coated with phenolic resin, which can be converted to carbon through a pyrolysis heat-treatment. The carbon then reacts away the silica coatings on the particles through reduction of silica, forming more SiC as a reaction product. Furthermore, boron carbide is known as a sintering aid for SiC, in which it is believed to increase grain boundary diffusivity during sintering heat-treatment. Solid-state pressureless sintering with these additives has been known to yield relative densities in excess of 95%.
Attempts to process solid-state sintered SiC to theoretical density by pressureless sintering to a closed-porosity state, followed by hot isostatically pressing (HIPing) of the part with no encapsulant has proven elusive. HIPing, while increasing the relative density of such sintered specimens, does not bring the specimens to their full theoretical density.